1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode, and more particularly, a large-sized high efficiency nitride semiconductor light emitting diode which can be used suitably in a high power lighting system.
2. Description of the Related Art
As well known in the art, nitride semiconductors in the form of III-V group semiconductor crystals such as GaN, InN and AlN are widely used in Light Emitting Diodes (LEDs) for emitting single wavelength light (e.g., ultraviolet ray and green light), in particular, blue light. Because a nitride semiconductor LED is fabricated on an insulation substrate such as a sapphire substrate and a SiC substrate satisfying lattice matching conditions for crystal growth, it necessarily has a planar structure in which two electrodes connected to p- and n-doped nitride semiconductor layers are arranged substantially horizontally on the top of a light emitting structure.
A planar LED has drawbacks that an effective light emitting area is not sufficient and luminous efficiency per light emitting area is low because the flow of electric current is not uniformly distributed across the light emitting area unlike a vertical LED in which both electrodes are arranged on the top and the bottom of its light emitting structure. An example of the planar LED and the restricted luminous efficiency will be described with reference to FIGS. 1a and 1b. 
FIGS. 1a and 1b illustrate an example of a conventional nitride semiconductor LED 10.
The nitride semiconductor LED 10 shown in FIG. 1a has p- and n-electrodes 19 and 18 both of which are arranged in diagonal corners on the top of a substantially rectangular LED body. Then, the conventional nitride semiconductor LED 10 has a planar structure with the p- and n-electrodes 19 and 18 being horizontally arranged side by side.
Describing it in more detail with reference to FIG. 1b illustrating a longitudinal section taken across a line A–A′ in FIG. 1a, the nitride semiconductor LED 10 has an n-doped nitride semiconductor layer 12, an active layer 14 and a p-doped nitride semiconductor layer 16 formed on the substrate 11 one atop another in their order on a sapphire substrate 11. As in this illustration, the p-doped nitride semiconductor layer 16 may be covered with a transparent electrode layer 17 such as tin-doped indium oxide or Indium Tin Oxide (ITO) in order to improve contact resistance.
Because the sapphire substrate 11 in use for the formation or growth of the nitride semiconductor layers is electrically insulated as described above, both the p-doped nitride semiconductor layer 16 and the active layer 14 are partially removed to form the n-electrode 18 that is to be connected to the n-doped nitride semiconductor layer 12. Owing to the electrical insulation of the substrate for growing the nitride semiconductor, the nitride semiconductor LED 10 has the planar structure with the p- and n-electrodes 19 and 18 being arranged on the same side.
In the planar semiconductor LED 10 shown in FIGS. 1a and 1b, current flow is concentrated on the shortest path between the both electrodes to narrow the current path which current density is concentrated on unlike the vertical LED allowing vertical current flow. Also, the current flow is directed laterally to increase drive voltage owing to large series resistance, resultantly reducing actual light emitting area. That is, the nitride semiconductor LED has drawbacks of low current density per unit area originated from limitations of the planar structure as well as low area efficiency owing to small light emitting area. As a result, it has been regarded very difficult to obtain a high power LED in use for lighting systems by a large-size (e.g., 1000×1000 μm).
In order to alleviate these problems, various forms of conventional approaches such as p- and n-electrode configurations and arrangements for raising current density and area efficiency have been developed as shown in FIGS. 2 to 3b. 
FIG. 2 is a plan view of an LED having an n-doped nitride semiconductor 22, an active layer and a p-doped nitride semiconductor layer (not shown) which are laid on a substrate one atop another in their order. On the top of the LED, p- and n-electrodes 29 and 28 are formed, connected to the p-doped nitride semiconductor layer (or a transparent electrode layer 27 if any) and the n-doped nitride semiconductor layer 22. The n-electrode 28 includes two contact pads 28a and a number of electrode fingers 28b extended from the contact pads 28a, respectively, and the p-electrode 29 includes two contact pads 29a and a number of electrode fingers 29b extended from the contact pads 29a, respectively, in which the n-electrode fingers 28b alternate with the p-electrode fingers 29b. This electrode structure can provide separate current paths through the electrode fingers 28b and 29b to reduce the lateral mean distance between the electrodes. This as a result can reduce series resistance, improve the uniformity of electric density across the whole area as well as ensure a sufficient light emitting area to the entire top surface even in case of a large-sized LED.
However, there is a problem that distal ends of the respective electrode fingers 28b and 29b show a lower optical power than other proximal portions thereof because they are placed substantially away from the contact pads 28a and 29a through which electric current is introduced.
FIGS. 3a and 3b illustrate a nitride semiconductor LED 30 having another conventional electrode structure. Referring to FIG. 3a, a p-electrode 39 includes a contact pad 39a formed in a substantially central area on the top of the LED 30 and four electrode fingers 39b extended from the contact pad 39a in diagonal directions. An n-electrode 38 includes a contact pad 38a formed adjacent to a corner on the top of the LED 30, an extension 38b extended from the contact pad 38a along adjacent to the outer periphery to surround the p-electrode 39 and four electrode fingers 38c extended from the extension 38b toward the p-electrode contact pad 39a. 
As shown in FIG. 3b, the nitride semiconductor LED 30 has a light emitting structure which includes an n-doped nitride semiconductor layer 32, an active layer 34 and a p-doped nitride semiconductor layer 36 formed on a substrate 31 one atop another in their order. On the top of the light emitting structure, a transparent electrode 37 may be formed on the p-doped nitride semiconductor 36 to improve the contact resistance with the p-electrode 38. Herein, both the n-electrode contact pad 38a and the n-electrode extension and 38b are formed on the n-doped nitride semiconductor layer 32 exposed along the outer periphery of the LED 30, and both the p-electrode contact pad 39a and the p-electrode fingers 39b are formed on the transparent electrode 37 and electrically connected to a p-doped nitride cladding layer 37.
In the nitride semiconductor LED 30 shown in FIGS. 3a and 3b, because the contact pads and terminals of other electrode regions are formed shorter than in the structure shown in FIG. 2 and the both electrodes are distributed at a uniform gap across the entire area, series resistance can be reduced to improve luminous efficiency and current density can be distributed uniformly.
However, because the active layer is removed by a considerable amount in order to form the n-electrode, this electrode structure also has drawbacks in that actual light emitting area is remarkably reduced with respect to the whole size of the originally grown light emitting structure and luminous efficiency per unit area is degraded on the contrary according to the size growth of the LED.
As a consequence, novel electrode structures and arrangements for ensuring higher power to large-sized nitride semiconductor LEDs have been incessantly searched in the art.